The invention relates to a rotary or linear electric motor whose armature is driven by piezo-electric vibration elements which are made to resonate by means of high-frequency electrical oscillations this produces ultrasonic vibrations which are transmitted to a plurality of drive pins arranged as brushes and against whose free ends a friction surface of the armature is pressed, the individual drive pins being disposed at an angle .alpha. relative to the normal to the contact portion of the friction surface.
Such a piezo-electric motor is disclosed in U.S. Pat. No. 4,453,103. By applying a high-frequency alternating voltage to the piezo-electric vibration element mechanical vibrations are set up in this element, which vibrations are transmitted to the armature via the transmission pins. The amplitude of the mechanical vibrations is a few .mu.m. The transmission pins, which may be arranged on the vibration element in the same way as brushes, transmit the mechanical vibration of the vibration element to the armature, each vibration causing the armature to be advanced by a corresponding number of .mu.m. Each individual transmission pin can exert only a small translatory or rotatory force on the friction surface of the armature. The friction area between the rotor and the transmission pins is only a few square millimetres. Such a small friction area results in the transmission pins being subjected to substantial mechanical loads, which may lead to rapid wear.
In the construction described in U.S. Pat. No. 4,453,103 the transmission pins are set at an angle .alpha. relative to a perpendicular to the friction surface of the armature. The setting angle must have values between 40.degree. and 50.degree.. However, such a setting angle results in a low efficiency of the motor.
It is further known from U.S. Pat. No. 4,325,264 to make the angle at which the transmission pins are set very steep, for example 5.degree.. This motor also has a low efficiency. Moreover, it can produce only comparatively small torques and tends to produce a substantial amount of noise under mechanical load.